The invention relates generally to a method and apparatus for filtering pulsed RF signals. More particularly, the invention relates to a method and apparatus for improving the frequency selectivity of a linear receiver by rejecting large, out-of-band, pulsed RF signals while preserving a large instantaneous dynamic range of the receiver.
In known linear receivers, pulsed RF signals are normally sorted in frequency by bandpass filtering. However, satisfactory rejection of large, out-of-band, pulsed RF signals may not be possible in receivers having a large instantaneous dynamic range through the use of bandpass filters. When large out-of-band pulsed RF signals occur, there may be sufficient sidelobe energy in receiver bandpass filter to cause detection. In some receivers, the energy within the filter passband is used for frequency discrimination; thus, the frequency of the out-of-band signal will also appear as if it is in-band.
In the past, various types of guard band filtering have been used in RF receivers to reject large, out-of-band signals. For example, U.S. Pat. No. 3,218,556, issued Nov. 16, 1965 to Chisholm describes a spectrum centered receiver which passes the bandpass frequencies of interest to the output only when an auxiliary receiver detecting signals on bands adjacent to or on either side of the main passband detects an energy level less than that of the main passband. U.S. Pat. No. 3,953,802, issued Apr. 27, 1976 to Morris et al, describes an adjacent channel rejector for a receiver in which the output of a high gain amplifier receiving an incoming signal is applied to both a wideband filter/demodulator and a narrowband filter/demodulator. The amplitudes of the signals from these two filters/demodulators are compared, and the output of the main receiver is inhibited whenever the wideband signal is greater than the narrowband signal. U.S. Pat. No. 3,621,401, issued Nov. 16, 1971 to Young, Jr., describes a frequency spectrum responsive noise reduction system for a receiver in which the output of a wideband regulating amplifier receiving an incoming signal is supplied to both a notch filter and a pass filter. The outputs of these filters are detected and the amplitudes compared with preadjusted reference threshold levels, and the signal of the main receiver is only allowed to pass the two receiver output when the amplitudes of the signal from both filters fall within satisfactory ranges.